Types of Immunities Found in Organism: Non-Specific and Specific Immunity

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Types of Immunities Found in Organism: Non-Specific and Specific Immunity!

Immunity is of two types: innate and acquired.

A. Innate Immunity (Non-Specific Immunity):

Innate immunity is the resistance to infection, which an individual possesses by virtue of his/her genetic and constitutional make up.

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Thus innate immunity comprises all those defines elements with which an individual is born, and which are always available to protect a living body. One form of innate immunity comprises various types of barriers which prevent entry of foreign agents into the body.

When pathogens enter into the body, they are quickly killed by some other components of this system. This is the first line of defence of most animals and plants. Innate immunity consists of the following four types of barriers: physical, physiological, cellular and cytokine barriers.

1. Physical Barriers:

These barriers prevent the entry of organisms into the body.

(a) Skin:

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The skin is physical barrier of body. Its outer tough layer, the stratum corneum prevents the entry of bacteria and viruses.

(b) Mucous Membrane:

Mucus secreted by mucous membrane traps the microorganisms and immobilises them. Microorganisms and dust particles can enter the respiratory tract with air during breathing which are trapped in the mucus. The cilia sweep the mucus loaded with microorganisms and dust particles into the pharynx (throat). From the pharynx it is thrown out or swallowed for elimination with the faeces.

2. Physiological Barriers:

Body temperature, pH of the body fluids and various body secre­tions prevent growth of many disease causing microorganisms. Some of the important examples of physiological barriers are as follows:

(d) Lysozyme is present in tissue fluids and in almost all secre­tions except in cerebrospinal fluid, sweat and urine. Lysozyme is in good quantity in tears from eyes. Lysozyme attacks bacteria and dissolves their cell walls,

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(e) A rise of temperature (fever) due to infection is a natural defence mechanism and helps not only to accelerate physiological processes but may, in some cases, destroy the infecting pathogens,

(f) Certain kinds of cells, when infected with a virus, release interferons (glycoproteins). Interferons (IFNs) make the cells resis­tant to viral infections, (g) Bicarbonate ions in saliva neutralise the acids in food.

3. Cellular Barriers:

Certain types of leukocytes (WBC) like polymorphonuclear leucocytes (PMNL), neutrophils and monocytes and natural killer (type of lymphocyte) in the blood and macrophages in tissues can engulf microbes, viruses and cellular debris etc.

The phenomenon of phagocytosis was discovered and named by Metchnikoff (1883). He proposed the phagocytic response as the prime defence against the microbial invasion of tissue. Metchnikoff and Paul Ehrlich got the 1908 Nobel Prize for their work on body resistance.

4. Cytokine Barriers:

Virus infected cells secrete pro­teins known as interferons which protect non infected cells from further viral infection. Fever may be brought about by toxins produced by pathogens and a protein called endogenous pyrogen (fever producing substance), also called interleukin released by macrophages. When enough pyrogens reach the brain, the body’s thermostat is reset to a higher temperature, allowing the temperature of the entire body to rise.

Mild fever strengthens the defence mechanism by activating the phagocytes and by inhibiting the growth of microbes. A very high temperature may prove dangerous. It must be quickly brought down by giving antipyretics. In addition to the above mentioned barriers, natural killer cells and the complement system also provide innate immunity.

Natural Killer Cells (NK Cells):

Besides the phagocytes, there are natural killer cells in the body which kill virus-infected and some tumour cells. Killer cells produce performing which create pores in the plasma membrane of the target cells. These pores allow entry of water into the target cells, which then swell and burst. Cellular remains are eaten by phagocytes.

The Complement System:

The complement system is a defensive system consisting of plasma proteins that attack and destroy the microbes. The term ‘complement’ (c) refers to a system of factors occurring in normal serum that are activated characteristically by antigen antibody interaction, and subsequently mediate a number of biologically significant consequences.

This system participates in both innate and acquired immunities. The complement system consists of over 30 proteins that act in various ways to protect the individual from invading microbes. Complement proteins create pores in the plasma membrane of the microbes (Fig. 4.4). Water enters the microbes.

The latter burst and die. Some components of the complement system form a coat over the invading microbes. This coating attracts phagocytes (neutrophils and macrophages) for engulfing them. The complement system also causes agglutination of microbes, neutralisation of viruses, activation of mast cells and basophils and has some inflammatory effect.

Second Line of Defence:

Phagocytes, interferon’s, inflammatory reactions, fever, natural killer cells and complement system constitute the second line of defence. Third line of defence is provided by specific defence mechanism which includes (i) antibodies and (ii) lymphocytes to be discussed ahead.

B. Acquired Immunity (= Adaptive or Specific Immunity):

The resistance that an individual acquires during life, is called acquired immunity. Acquired or adaptive or specific immunity has the following properties:

(i) Specificity:

It is the ability to differentiate between various foreign molecules.

(ii) Diversity:

It can recognise a vast variety of foreign molecules.

(iii) Discrimination between Self and Non-self:

It can recognise and respond to foreign molecules (non-self) and can avoid response to those molecules that are present within the body (self) of the animal.

(iv) Memory:

When the immune system encounters a specific foreign agent, (e.g., a microbe) for the first time, it generates immune response and eliminates the invader. This is called first encounter. The immune system retains the memory of the first encounter. As a result, a second encounter occurs more quickly and abundantly than the first encounter.

Cells involved in Acquired Immunity:

Lymphocytes:

A healthy person has about a trillion lymphocytes. Lymphocytes are of two types: T lymphocytes or T cells and B lymphocytes or B cells. Both types of lymphocytes and other cells of the immune system are produced in the bone marrow. The process of production of cells of immune system in the bone marrow is called haematopoiesis.

(i) T Lymphocytes (= T cells):

Certain stem cells in the bone marrow give rise to immature lymphocytes. These lymphocytes migrate via blood to the thymus. Once these cells enter the thymus, they are called thymocytes. In the thymus these cells mature as T lymphocytes (T cells).

Types of T-Cells and their functions:

(a) Helper T cells. They are numerous. These cells stimulate the B-cells to produce antibodies. They also stimulate the killer T cells to destroy the non-self cells. Their role is overall regulation of immunity. They do this function by forming a series of protein mediators, called lymphokines that act on other cells of the immune system as well as on bone marrow cells,

(b) Cytotoxic T cells (= Killer Cells or K Cells). These cells directly attack the foreign cells. The cytotoxic T cells secrete a protein perforin which punctures the invader’s cell membrane. Water and ions flow into the nonself cell, which swells up and finally lyses. The cytotoxic T cells also destroy the cancer cells. The cytotoxic cells are responsible for cell mediated immunity,

(c) Suppressor T Cells. They are capable of suppressing the functions of cytotoxic and helper T cells. They also inhibit the immune system from attacking the body’s own cells,

(d) Memory T Cells. These cells remain in the lymphatic tissue (e.g., spleen, lymph nodes) and recognize original invading antigens, even years after the first encounter. These cells keep ready to attack as soon as the same pathogens infect the body again.

(a) Plasma Cells (Effector B cells). Some of the activated B cells enlarge, divide and differentiate into a clone of plasma cells. Although plasma cells live for only a few days, they secrete enormous amounts of antibody during this period. A few days after exposure to an antigen, a plasma cell secretes hundreds of millions of antibodies daily and secretion occurs for about 4 or 5 days until the plasma cell dies,

(b) Memory B Cells. Some activated B cells do not differentiate into plasma cells but rather remain as memory cells. They have a longer life span. The memory cells remain dormant until activated once again by a new quantity of the same antigen.